There is described a method for heat exchanging in and work exchanging with a working fluid in a heat engine or a heat pump if the method and its sub-processes are essentially reversed, wherein a thermodynamic cycle for the working fluid is approximately described through the polytropic relation PVn=constant, P representing the pressure, V representing the volume and n representing the polytropic index of the working fluid having adiabatic index gamma (γ), and wherein the engine consists of at least one work mechanism provided with a first and at least a second volume change chamber.
There is also described a heat engine for use in exercising the method.
Recently there has been substantially increased focus on the utilisation of renewable energy sources. There are many forms of renewable energy available, most of the available, renewable energy being in the form of heat, and, in the end, water energy, wind energy and parts of the ocean energy are products of solar irradiance and thus results of heat energy or thermal energy which is a more formal term.
Thermal energy may be utilised directly, for example to heat water, but generally there is a need to convert the energy to a different form that may be utilised for purposes other than heating. The best example is electrical energy that may be produced by means of a thermal energy engine, also called a thermodynamic engine, or most plainly called a heat engine, which is a more general term. A heat engine is in most cases a mechanical device, which can utilise the temperature difference between a heat reservoir and a cold reservoir to produce mechanical work. From mechanical work, energy in the form of such as electricity may be further produced.
Examples of heat engine types are steam engines, petrol engines, diesel engines, Stirling engines, gas turbines and steam turbines (also called Rankine turbines, which among other places are used in most coal fired power plants and nuclear power plants). There exist many more types. Petrol and diesel engines and also gas turbines are characterised as internal combustion engines, as the heat energy for these is obtained by internal combustion of fuel. Steam engines and Stirling engines utilise heat from external combustion and are therefore often called external combustion engines.
The term external combustion engine may often be misleading as the heat energy for a so-called external combustion engine may just as well come from the sun or another form of heat source not requiring combustion of a fuel. Another example of a heat source without combustion is geothermal heat or ground heat as it is also called. This heat is latent in the earth's crust or even deeper. The term external combustion engine may therefore advantageously be replaced by external heat engine or engine with external heat supply, which is a more appropriate term.
With new international requirements regarding reduction of greenhouse gas emissions and also use of non-renewable energy sources, there turns out to be a strongly increasing need for renewable energy sources. In this connection there is also a growing need to be able to utilise heat at lower temperatures, such as from geothermal wells or solar energy plants. An important observation here is that the lower the source temperature the more energy is available and the cheaper it is to procure. The available heat energy may be divided into for example two groups defined as low-grade and high-grade heat energy, low-grade energy being defined as heat having a temperature below what may be utilised in traditional steam turbines, which for some technologies start at such as 150° C., while other technologies utilise temperatures from 300° C. High-grade heat sources then have typical temperatures above this. The drawback in utilising heat energy at low temperatures is that the theoretical maximum for the efficiency is low, but as long as there is enough energy available, this is less important.
Nevertheless, one may get improved utilisation of the total available energy by combining different energy sources, for example by supplementing low-grade heat energy with high-grade heat energy, the total efficiency being relatively high, without all the heat needing to come from an “expensive”, high-grade reservoir.
Today there are several technologies that in several cases solely use low-grade heat sources. Examples of these are Stirling engines and “Organic Rankine Cycle” turbines, so-called ORC turbines. ORC turbines follow the Rankine cycle just like traditional steam turbines, but instead of water they often use an organic working fluid having a low boiling point at atmospheric pressure such as pentane (boils at 36° C. at 1 atmosphere), diethyl ether or toluene, hence the name part “Organic”. By using a fluid having a low boiling point, heat energy at temperatures well below 100° C. (normal boiling point for water) may be utilised.
Current low temperature technology has a few drawbacks, giving large room for further improvement. ORC solutions require for example relatively advanced turbine technology, making this technology less available in areas where the technical expertise is low, the use of this technology furthermore entailing large costs. ORC plants require in addition large evaporator tanks, as the working fluid for the ORC turbines ideally speaking must be evaporated completely before it enters the turbine itself, thus requiring large volumes for heat exchangers. If this is not satisfied, one may in several types of turbines get blade erosion as a result of the large forces that presence of liquid in the turbine may involve. If the blades in a turbine erode, it will be destroyed. In addition, turbines are generally adiabatic, that is to say no heat is added during the expansion, contrary to such as Stirling engines where a near isothermal (or more real polytropic) expansion takes place. The Stirling technology has also several challenges turning out to be difficult to solve, large demands being made on inter alia material properties and heat exchangers, where materials and the remainder of components required for Stirling engines are not normally found as standard goods within the most common engine industries. This makes the Stirling technology very costly, and advanced expertise is required for production and maintenance in the use of this technology.